The present invention relates to a vapor deposition apparatus and method for applying a coating to a base material, and more particularly to a vapor deposition apparatus and method wherein small amounts of refractory elements are incorporated into a vapor deposition coating.
The thermal evaporation and condensation of solid materials such as metals to form a coating on a base material, commonly referred to as vapor deposition, is a relatively developed art. There are many sophisticated prior art techniques and apparatuses which permit such materials to be evaporated from a source and condensed to form a coating or layer on a substrate disposed a distance from the source. Such processes all involve heating a material to be evaporated to a temperature at which it has a significant vapor pressure, thus creating a vapor stream. Heating techniques include direct methods, such as heating the material to be deposited using resistance, induction, electron beam or laser beam means to melt all or some portion of the material to be evaporated, or indirectly, such as by heating the surface of a higher melting material and flashing the material to be evaporated off the hot surface. The evaporated material thereafter becomes condensed on the surface of the base material, thereby providing a coating thereon.
Refractory materials are often desired to be incorporated into coatings applied to the surface of components exposed to high temperatures, such as gas turbine components used inter alia in aircraft engines, to act as a protective coating in a process known as thermal barrier coating (TBC). Methods for depositing ceramic barrier coatings like zirconia (i.e. zirconium oxide) which serve as thermal barrier coatings are known in the art. For example, U.S. Pat. No. 5,773,078 to Skelly, commonly assigned to the assignee of the present invention, namely General Electric Company, discloses an improved method for depositing zirconium oxide from a zirconium oxide source onto a base material by means of physical vapor deposition, comprising the step of adding zirconium metal to a zirconium oxide ingot as the ingot is heated. Despite the improvements realized by the method of U.S. Pat. No. 5,773,078, use of a rare earth metal oxide such as zirconium oxide as an evaporant source causes problems due to release of the oxygen present in the oxide and difficulties in regulating the uniformity of composition in the resulting deposited condensate.
Where the starting material to be evaporated and applied as a coating to a base material is a multi-constituent alloy, other problems are encountered. In particular, the composition of the coated material when applied by the vapor deposition method as described frequently and undesirably was substantially different than the composition of the starting material, and/or the condensate would not have a uniform composition through its thickness which closely resembled that of the starting material. These problems were directly due to the fact that the rates of evaporation of elements contained in the multi-constituent starting material alloy are related to their vapor pressures at the temperature of the evaporation source. In the case of alloys, particularly multi-constituent alloys, one or two elements thereof typically have significantly higher vapor pressures than the others, such that the condensate is richer than the starting material in these elements. If the material being evaporated has a fixed volume and is entirely evaporated, the condensate will have a non-uniform composition throughout its thickness, but will reflect, in a macroscopic sense, the starting composition of the material. If the starting material is continually replenished, such as by maintaining a constant pool volume, the composition of the condensate will be higher throughout its thickness in the elements which have higher vapor pressures.
U.S. Pat. No. 5,474,809 to Skelly et al, assigned to General Electric Company who is the common assignee with respect to the present invention, expressly recognized the problems of the prior art in achieving uniform and desired composition for the coating closely corresponding to that of the evaporated material. Such patent disclosed a method for carrying out vapor deposition that achieved a coating on a base material which closely resembled the composition of the starting (i.e. evaporated) material, that is to say the coating purportedly to contain substantially the same elements in substantially the same proportions as the starting material, even if the starting material was a multi-constituent alloy having elements each of significantly different vapor pressures. In particular, the aforementioned Skelly et al patent disclosed a method of making an evaporated deposit of a material using the vapor deposition process, wherein one material (a second material) having a composition which was desired to be formed as a coating on a base material, is overlaid by a first material which consisted of a refractory material with a higher melting point or a vapor pressure at an elevated temperature that is less than each of the constituents of the second material. Accordingly, upon heating of the first material the underlying second material becomes melted, and the constituent elements of the second material proximate the overlying first material are transported by convection and thermal mixing through the first overlying material and thereafter evaporated from the surface of the first overlying material. In such process the first material becomes molten and transmits heat downwardly to the underlying second material, thereby forming a molten zone therein, and second material in such molten zone therein becomes mixed with the molten zone of first material immediately above it, permitting the second material to be evaporated from the surface thereof. Advantageously, such process purportedly permits coatings to be formed on a base material having a composition which is substantially identical to that of the second material. The second material (or at least certain of the elements therein) which were desired to be evaporated possessed vapor pressures which permitted such elements to be preferentially evaporated in comparison to the first material. Accordingly the deposit contained quantities of the second material, but no or only minute trace amounts of the first material (less than 0.05 atomic percent).
In the case of refractory materials in the form of rare earth materials such as zirconium or hafnium intended to be incorporated into a thermal coating for deposit on a base material, it is actually desirable for such materials to be evaporated and thereby incorporated in the deposited coating where a thermal barrier coating is desired to be applied. However, when rare earth metals, such as zirconium or hafnium are used in the process of Skelly et al described in U.S. Pat. No. 5,474,809 as the first material, and metal alloys such as a nickel-aluminum alloy is used as the second material, it is found that the method taught by Skelly et al is physically unworkable. In this regard, when employing a rare earth metal, such as zirconium, as the first material using the method taught by Skelly et al, such first material when melted tends to “ball-up” when heated by a heat source such as an electron beam, by virtue of the surface tension forces existing between molten zirconium and the solid second material. As a result there is little or no proper transfer of heat downwardly to the underlying second material to form a molten zone there within so as to permit molten second material to migrate upwardly there through and thereafter evaporate from the surface. In such circumstances, neither the underlying second material or the zirconium which comprises the first material becomes evaporated so as to form a deposit. Moreover, in Skelly et al even where the first material is not a rare earth metal, the Skelly et al patent did not develop or disclose any circumstances in which it was capable of obtaining quantities of the refractory metal in the deposit in concentrations greater than trace amounts (i.e. greater than 0.05 atomic percent).